In the United States, approximately 90,000 individuals are considered to be upper-body amputees, having lost all or a portion of an upper limb. Of those, a subset will use a prosthetic (artificial) arm to enhance or restore their capabilities and, hopefully, their quality of life. While a number of prosthetic devices have been developed to assist these individuals, their use is not wide-spread due, at least in part, to the poor performance and design of existing prosthetics and prehensors. A prehensor, also known as a “gripper” or an “end-effector,” is a mechanical grasping device used by an upper-body amputee to serve as an artificial hand.
In response to the need for prosthetic arms and associated prehensors, two types of devices have been developed and marketed: battery-powered electronic systems and body-powered mechanical systems. Electronic systems using a battery package, electric motors, and sophisticated electronic controls have been developed and shown to work reasonably well. Unfortunately, electronic systems are very expensive. In addition, electronic systems can be unrealistically heavy and suffer from inadequate battery life. U.S. Pat. No. 4,792,338 issued to Rennerfelt discloses an electronic or battery-powered prehensor.
The second type of prosthetic devices commonly used are called “body-powered” systems because the wearer controls the system using muscles in his or her body, usually muscles in the shoulder and neck. Body-powered mechanical systems are generally lighter, quieter, and far less expensive than their electronic counterparts, and do not suffer from battery-life limitations.
Within the realm of body-powered prosthetics, there are two primary families of prehensors, differing primarily in their principle of operation. Voluntary opening (VO) prehensors typically include two or more gripping digits (mechanical fingers with rubber pads for friction and better grip) that are held or biased against each other by a spring or one or more strong rubber bands. The wearer moves the digits apart prior to gripping by pulling on a control cable connected to the wearer's shoulder and neck through a harness. When the wearer relaxes or eases the tension on the control cable, the digits close on the object to be held and “grip” it. In essence, voluntary opening prehensors are spring loaded clamps that can be opened at will by the wearer. Therefore, with a voluntary opening prehensor, the wearer's grip on the object is passive and the wearer need do nothing to maintain grip.
Voluntary opening prehensors are popular due to their low cost as compared to electronic prehensors, and the fact that the wearer does not expend energy while gripping an object. Unfortunately, since gripping an object with a voluntary opening prehensor is passive, i.e., the wearer is not expending energy to maintain the grip, the wearer has limited, if any, control over the amount of force exerted on the object. Gripping forces needed to lift heavy objects are excessive for small or lightweight fragile objects. Conversely, the correct gripping force needed to lift a light object will usually be inadequate for heavier objects. U.S. Pat. No. 3,604,017 issued to Brown et al. and U.S. Pat. No. 5,116,386 issued to Scribner disclose voluntary opening prehensors.
The second major type of body-powered prehensors are the voluntary closing (VC) prehensors. As its name implies, unlike a voluntary opening prehensor, the gripping digits in a voluntary closing prehensor are closed upon an object to be grasped by actively exerting force on a control cable attached to the wearer's shoulder and neck using a harness. Voluntary closing prehensors offer several important advantages over voluntary opening prehensors. First, a voluntary closing prehensor is more physiologically intuitive than a voluntary opening prehensor. That is, a voluntary closing prehensor requires a wearer to exert muscular force to grasp and hold an object while a voluntary opening prehensor requires the wearer to relax his or her muscles to initiate and maintain a grip. Second, in a voluntary closing prehensor, the gripping force applied to the object to be grasped by the wearer is directly related to the force the wearer exerts on the control cable. Requiring the wearer to exert force when grasping an object provides feedback to the wearer, thereby giving the wearer a sense of how strong his or her grip is upon the object. This feedback, also called physiological proprioception, allows the voluntary closing prehensor to become an extension of the wearer's body with a natural feel and a confident grasp. Since voluntary opening prehensors do not provide this feedback, the wearer is effectively removed from the gripping cycle. Third, by requiring that the wearer only exert the amount of energy necessary to attain the gripping force required to grasp an object, voluntary closing prehensors conserve a large amount of the wearer's energy. In contrast, voluntary opening prehensors require the wearer to stretch springs or rubber bands to separate or open the digits each time grip is to be applied to an object, regardless of the size or weight of the object. Any excess energy used to open the digits is wasted.
While voluntary closing prehensors are generally more energy efficient than voluntary opening prehensors, voluntary closing prehensors still require the wearer to exert significant energy while maintaining grasp on an object. Therefore, wearers desire voluntary closing prehensors that reduce the energy needed to grasp an object as much as possible while providing feedback as to the force the wearer is exerting against the object. Many types of voluntary closing prehensors are known in the prior art. For example, U.S. Pat. No. 4,225,983 issued to Radocy et al. and U.S. Pat. No. 4,332,038 issued to Freeland both disclose voluntary closing prehensors. Radocy et al. focus their prehensor design towards achieving optimally configured gripping surfaces for the prehensor that can be inexpensively manufactured using stamped plate construction techniques. While Radocy et al. provide a locking pawl to assist the wearer in maintaining a grip on an object, unfortunately, Radocy et al. require that the wearer manually actuate and release the locking pawl, a known safety hazard. Freeland discloses an artificial hand with a pivotal thumb to adapt the hand for gripping different objects. Unfortunately, Freeland does not provide an energy efficient device capable of assisting a wearer in maintaining a grip on an object.
Despite the well developed state of the prior art, there remains a need for a voluntary closing prehensor that conserves the energy expended by the wearer to size and grip an object. Preferably, the voluntary closing prehensor will mechanically assist a wearer in maintaining the grip on the object without requiring any additional manual intervention by the wearer.
To understand the drive problem with current designs, a review of the natural, normal grasp is in order. In execution, the grasp cycle is comprised of two parts: 1) “sizing,” where the fingers are brought towards one another to wrap around an object, and 2) “gripping,” where force is applied to the object with the fingers to secure it within the hand and thereby permit manipulation. Individuals with their normal hand perform this two-step sequence intuitively, near instantaneously, and subconsciously, permitting the mind to remain focused on high-level aspects of the activity being carried out. Body-powered prosthesis users, in contrast, must consciously generate control cable tension by harnessing unrelated body motions (e.g. scapular abduction, shoulder elevation, elbow flexion); moreover, they can only maintain useful tension levels over relatively short cable excursions.
Average users can develop approximately 2 inches of cable travel and generate approximately 20 lbf cable tension repeatedly without excessive fatigue and discomfort; where tension must be sustained, however, such as for voluntary closing operation, lower values are preferred. Other investigators have suggested that prehensor designs enable users to hold objects up to 3 inches in diameter with tip-pinch forces approaching 12 lbf for reasonable “real world” performance.
A portion of the total available cable excursion must be allotted to each stage of the grasping cycle—sizing and gripping. For two inches of possible cable travel, half of the possible excursion, or 1 inch, is reserved for each part (altering this balance exacerbates the problem described below). In voluntary closing operation, this small excursion must swing the prehensor's moveable digit through its full sizing range of motion to bring it into contact with any size of object within the prehensor—from a sheet of paper to a large bottle—in preparation for the application of gripping force. For the commonly-used digit length of 3.5 inches, the maximum closing angle is computed:
                                          3.5            ″                    ⁢                      {                          2              ⁢              sin              ⁢                                                          ⁢                              θ                2                                      }                          =                  3.0          ″                                    Equation        ⁢                                  ⁢                  {          1          }                    Solving for the sizing angle, θ, we obtain 51°. The tip (where force is actually applied) of a 3.5-inch finger swinging through this angle follows an arc segment having length:
                              arc          ⁢                                          ⁢          length                =                                            3.5              ″                        ⁢                          {                                                                    (                                          51                      ⁢                      °                                        )                                    ⁢                  π                                180                            }                                =                      3.1            ″                                              Equation        ⁢                                  ⁢                  {          2          }                    This arc length is traversed as the input cable moves through its 1.0-inch excursion, giving an effective mechanical advantage, or forward force ratio (FFR), of 0.32 or less. This ratio emphasizes motion over force, ensuring the drive mechanism can fully close the prehensor from its full open position within the permissible cable excursion limit. Tip force at this ratio is limited to just 1/3 the input cable tension—a relatively low value.
For the second portion of the grasp cycle, gripping, the calculation is simpler. The permissible input tension, 20 lbf (acceptable for short-duration grasping with VC units), should optimally be transformed into at least 12 lbf of tip-pinch force, requiring a FFR of 0.60 or more. Higher mechanical advantage ratios would permit the same grip force to be reached while demanding less muscular exertion. These higher ratios, however, emphasize force over motion, and would hypothetically require nearly 5.2 inches of cable excursion to fully close the device—far beyond that which can be generated through normal harness motions.
A simple mechanical lever arrangement offering only a single, fixed mechanical advantage is inherently incompatible with these opposing functional requirements. That is, it is mechanically impossible for a drive mechanism using a single fixed lever to meet these dual operating requirements.
Some prior art devices attempt to compromise and skirt this functional problem using “average” lever ratios between 0.4 to 0.5, but this sets the device up to deliver poor to marginal performance in both grasping regimes. Fixed-lever systems of this type are attractive because they are mechanically simple and robust, and can be manufactured for low cost. Nevertheless, terminal devices using them are inherently inefficient designs.
Machine theory, i.e. the Chebychev-Grübler-Kutzbach Movability Criterion, requires another degree of freedom be incorporated into the drive mechanism if two mechanical advantages are to be realized-switching lever ratios between grasp cycle stages for example.
Existing prehensor devices are typically configured to provide either a voluntary opening mode of function or a voluntary closing mode of function. Voluntary closing devices, as the name implies, close as the user increases tension in the control cable actuating the unit. Voluntary opening devices, in contrast, also use an actuation cable, but close as the user relaxes their tension. With voluntary opening, cable tension vanishes when the user fully relaxes; grasp may be sustained, however, without fatigue. Large gripping forces and proprioceptive feedback make voluntary closing units desirable and intuitive to operate, but when not engaged in grasp, they are open with the digits apart, making them prone to strike doorframes, furniture, and the user's thigh. Moreover, existing individual voluntary opening or voluntary closing devices do not allow the wearer to changes modes between the functions. That is, wearers are left with a choice of using a voluntary closing mode prehensor, using a voluntary opening mode prehensor, or constant labor-intensive exchanges between two units. Accordingly, it would be advantageous to provide a single prehensor that can be switched back and forth between a voluntary opening and voluntary closing mode of function.
There is also a need for a safety device that releases the closed digits if the separation force on the digits becomes great enough. For example, if a user were to fall while wearing a prehensor gripped to a railing, the user may suffer significant bodily harm if the prehensor were to maintain grasp of the railing throughout the entire event of the user falling. Thus, it would be advantageous to provide a prehensor having a built in safety mechanism that automatically releases the prehensor digits, if necessary.
In addition, there is also a need for a prehensor cable that provides high tensile strength with low frictional characteristics that can be used on sheaves or pulleys with relatively small radii of curvature. Existing cables fabricated from steel or other metal alloys do not possess these qualities.
There is also a need for a prehensor that includes digits that are replaceable if a digit is damaged or the user desires to change it for a new one. More particularly, users of prehensor devices sometimes damage the digits of their prehensor device by, for example, scraping, bending, or burning at least a portion of a digit. Digits are also frequently and permanently soiled and degraded by common chemicals, such as gasoline and cleansers. Accordingly, a need exists to provide digits that can be replaced relatively easily.
There is also a need for the prehensor to incorporate self-sanitizing technology. More particularly, under a variety of circumstances, the digits of a prehensor may be exposed to or become contaminated with unsanitary matter. Accordingly, it would be advantageous to provide prehensor digits that incorporate a self-sanitizing feature.